In monitoring the blood pressure of a patient at some point in a blood vessel, it is customary to connect one end of a catheter to a pressure dome, fill the dome and catheter with a saline solution, and insert the other end of the catheter through the blood vessel until it reaches the point of interest. In order to prevent the catheter from being clogged with blood particles, a small flow of saline is maintained through it. When, on occasion, blood samples are obtained by drawing blood through the catheter, the catheter is cleaned out by flushing it with a fast flow of saline solution. In order to prevent injury to the patient, it is necessary that the fast flow be limited to brief intervals. Accordingly, devices have been designed that provide a continuous slow flow to prevent clogging and a fast flow to flush the catheter when activated by the clinician. The fast flow is terminated by the device after it is deactivated so that fail-safe operation is attained. When the dome and catheter are first filled with saline solution, the fast flow is used to as to reduce the time required. In order to obtain good pressure measurements, it is essential that the dome and catheter be free from bubbles; but with prior art devices, it is difficult to eliminate them.